crosh: fix bashism in command parsing

The string replace function only works with bash.

BUG=None
TEST=ran `crosh --dash` and verified commands still worked like 'ping localhost'

Change-Id: I15b666a3c47652fdc6daa5507bbd2eaedc7597f0
Reviewed-on: https://chromium-review.googlesource.com/178931
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed